Load indicators have been disclosed and described in GB 2 255 109 A having a closed grommet thimble, as well as in GB 2 223 102 A having a type of strain gauge.
Also known as disclosed in EP 0984 873 B1 is a one-piece load indicator in which the tensile forces can be determined by the variable distance of the leg ends of a spring which takes the form of a grommet thimble. Here the spring encloses a tie bolt which is located within a loop of the lashing-down strap and also describes a path which is easily discernible visually under small loads.
A disadvantage of this approach is that under high tensile forces the shape of the spring changes geometrically such that this increased force is no longer discernible visually but must be measured using an additional amplification.
All known measuring devices and load indicators of this type are located inside the loop of the tie-down strap or lifting sling. Load indicators of the same type and for the same technical means can be attached externally, thereby pressing on the outer faces of the loop. In this case, the inner faces of the loop are in contact at zero tension and are then forced further apart as the tension increases, in other words, exhibit a reversed functional principle.
The goal of the known load indicators is to measure the entire range of the load, and ought therefore to have the ability to accommodate both low and high forces through the spring force. In the existing devices, high forces are accommodated by using a heavy material or by a small movement in the spring elements. In order to be able to measure the low forces that occur in response to a pretension, a weak spring is therefore required which is at the same time able to withstand a high tensile force. Practical experience has demonstrated that weak springs undergo changes in shape when overtensioned, thereby displaying incorrect values from that point on. In solving the task at hand, the approach is therefore to limit oneself to the tension and to ignore any forces exceeding this value.